A radiation window is a physical structure that transmits incident radiation (e.g., gamma rays, x-rays, ultraviolet light, infrared radiation, alpha particles, beta particles, electrons, protons, neutrons, etc.) while blocking unwanted species (e.g., gases, liquids, mobile solids, visible light, other radiation, etc.). When the primary purpose of a such a structure is to selectively transmit certain radiation while blocking other radiation, the structure is often referred to as a “filter.” As used herein, the term “window” refers to all such radiation-transmitting structures, regardless of what species they are intended to block.
Radiation windows are typically employed in devices that produce, detect, and/or analyze radiation. By way of example, x-ray florescence (XRF) devices, energy dispersive spectroscopy (EDS) devices, and x-ray diffraction (XRD) devices, all of which provide information about the elemental and/or structural composition of a material specimen by analyzing x-rays emitted from the specimen after it has been subjected to irradiation, typically employ an x-ray detector encased in a protective housing with a radiation window that allows the x-rays to penetrate the housing and reach the detector. In such applications, the radiation window is commonly referred to as an “x-ray window.” Common examples of x-ray detectors used in such applications are silicon drift detectors (SDD), quantum dot detectors (QDD), silicon-lithium (SiLi) detectors, and PIN diodes. Such detectors must typically be cooled substantially below room temperature to reduce electronic noise and improve performance. To protect the detector from degradation caused by environmental contaminants, the detector is typically sealed inside the protective housing under high vacuum or, alternatively, filled with a small amount of gas under partial vacuum. The vacuum or partial vacuum inside the detector housing is also important to minimize the attenuation of low-energy x-rays (often referred to as “soft x-rays”), which are easily absorbed by gas molecules.
There are many other applications for radiation windows, but two competing requirements common to most of them are that the windows must be thin enough to transmit the desired radiation with as little absorption or attenuation as reasonably possible while at the same time being robust enough to withstand whatever forces may be exerted on the windows (by differential pressures, mechanical vibrations, accelerations, etc.) without breaking or otherwise losing integrity, such as developing cracks or fissures that allow unwanted gases, radiation, or other species to leak through the window. These two competing requirements become increasingly problematic when the desired radiation is easily absorbed by any kind of solid matter, such as the case of soft x-rays emitted from irradiated “light elements” (i.e. elements of low atomic number, such as Li, B, C, N, O, and F), which have difficulty penetrating even extremely thin—and therefore very fragile—windows.
Thin radiation windows are usually made of materials composed primarily of relatively light elements, since such elements are typically less absorptive, and thus more transmissive, of weakly-penetrating radiation. Thin window materials used in the prior art include beryllium, aluminum, diamond, mica, quartz (silicon dioxide), boron, boron hydride, boron hydride alloy, boron nitride, silicon nitride, and polymers such as polyimide, polypropylene, polyethylene, polyester, polycarbonate, poly-vinyl formal (commonly referred to by the former trademark Formvar), poly-paraphenylene terephthalamide (commonly referred to by the registered trademark Kevlar®), etc. The window materials are fashioned into thin foils or films (all of which are referred to herein as “films”) which are attached across an opening in a more mechanically robust structure or housing (hereinafter referred to as a “window housing”). Polymers are often the film material of choice for extremely thin radiation windows (on the order of a few microns or less), primarily because they are less dense —and therefore more transmissive —than most other window materials, and because thin polymer films are typically less brittle than similarly transmissive films of other window materials. However, because thin polymer films are very permeable to gas molecules, they must be coated with a gas barrier layer (for example, a few hundred angstroms of aluminum) for applications which require a gas-tight window, such as the x-ray detectors mentioned above. Polymer films may also require thin coatings of non-polymeric materials for other purposes, including radiation filtration (such as a metallic layer on an x-ray window to filter out unwanted ultraviolet, visible, and/or infrared radiation) and electrical properties (such as a thin metallic coating to provide electrical conductivity on windows used in “proportional counter” radiation detectors).
In many applications, especially those in which the window must withstand substantial forces acting on it—such as where there is atmospheric pressure on one side of the window and vacuum on the other side—it may not be feasible for a free-standing film of the window material to span the opening in the window housing. In such situations, it is customary to employ a support structure, such as a rigid mesh or grid, to provide mechanical support for the window film. The primary design goals for such a support structure are to provide the requisite mechanical strength and rigidity to support the window film while interfering as little as possible with the transmission of the desired radiation.
As illustrated by way of examples in Prior Art FIGS. 1-4, support structures come in many different geometries and configurations, but common to all of them is a transmissive area 5 comprising a pattern or array of solid members 3 (hereinafter “support members”) to support the window film 4, and corresponding apertures 6 to allow the radiation to pass through the support structure. Configurations of support members and corresponding apertures used in the prior art include arrays of straight ribs and slots, round holes, polygonal holes (hexagons, rectangles, squares, triangles, etc.), and combinations of these. As suggested by the multiple reference lines for the support members 3 in the above-referenced Figures, the term “support member” as used herein refers to each individual segment making up the pattern or array of solid members supporting the window film, and not to the pattern or array as a whole.
Support structures also typically have a flange 2 peripheral to the transmissive area 5 for the purpose of attaching the support structure to the window housing 1. It should be noted that the flange may also be transmissive of radiation, but as a general rule the flange will transmit to a lesser degree than the transmissive area.
In the prior art, support structures have been made of relatively rigid materials such as silicon, quartz (silicon dioxide), diamond, boron, boron hydride, boron nitride, silicon nitride, and various metals including nickel, tungsten, molybdenum, stainless steel, aluminum, beryllium, and copper.
The inherent drawback of support structures is that they inevitably obscure a portion of the incident radiation, thus decreasing the overall transmission or performance of the window. Another potential drawback is that the material of the support structure itself, when exposed to the incident radiation, may be induced to emit radiation of its own which could contaminate the spectrum of the radiation passing through the window. These can be substantial drawbacks in applications where the quantity and/or spectral purity of the transmitted radiation are of concern.
One obvious way to increase the transmission of radiation through a given support structure is to modify the design of the support members and/or apertures so as to increase the fractional open area (i.e., the aggregate area of the apertures divided by the total area). However, this strategy can only be carried so far, since it eventually leads to a support structure which no longer has enough strength and/or rigidity to perform its critical function of supporting the window film.
Another way to increase the transmission of radiation through a support structure is to decrease the thickness of the support structure itself, thus decreasing the amount of radiation absorbed by the support members. This strategy can be particularly beneficial in applications where the window is intended to transmit radiation of varying energies or wavelengths, such as in typical XRF, EDS, or XRD systems, because although the less-penetrating radiation may still be completely absorbed (and therefore obscured) by the support members, a higher percentage of the more-penetrating radiation can potentially be transmitted through the support members (and therefore only partially obscured by them). Once again, however, this strategy can only be carried so far, since it also eventually leads to a support structure which no longer has enough strength and/or rigidity to perform its critical function of supporting the window film.
A third way to increase the transmission of radiation through the support structure is to select a material for the support structure which is less absorptive of the incident radiation. This strategy can also address the problem of spectral contamination, since a material which is less absorptive of the incident radiation is also less likely to become excited by it and induced to emit radiation of its own. However, this strategy is quite problematic, since materials which are less absorptive are also typically less mechanically robust and rigid. For example, support structures made of polymers have been proposed (see U.S. Pat. No. 5,578,360), since polymers are less brittle and more transmissive than other materials currently in use, but their lack of rigidity has prevented them from being seriously adopted. Simply put, if the support structure flexes too much, it results in failure of the window film.